With the development of the food industry, flexible packaging has become more and more important. The quality of flexible packaging is closely related to the adhesive used, and polyurethane adhesives, due to their excellent properties, can be used for flexible packaging materials with various properties such as cold resistance, oil resistance, drug resistance, transparency, and abrasion resistance. Therefore, it plays an important role in adhesives for flexible packaging. The status. Polyurethane adhesives are divided into two components and one component. One-component polyurethane adhesives generally consist of an isocyanate-terminated prepolymer or its closure, and are produced by reaction with an active hydrogen-containing component or moisture in the air to cure; a two-component polyurethane adhesive consists of a terminal hydroxyl group ( Polyester or polyether polyols or their polyurethane-type hydroxy-terminated products formed together with diisocyanates, chain extenders, and isocyanate-based components. When the terminal hydroxyl group is a polyester polyol, it has high heat resistance, adhesive strength, oil resistance, and oxidation resistance, but its hydrolysis stability is poor; when the terminal hydroxyl group is a polyether polyol, it is better. The flexibility, low temperature resistance and water resistance, but poor strength and aging resistance.
1 Water-based polyurethane adhesive
Water-based polyurethane adhesives were developed in response to the requirements of low toxicity and low pollution products in the 1970s. Water-based polyurethane adhesives are mainly prepared by the following two methods: First, the original polyurethane adhesive is directly emulsified into a stable water-dispersed emulsion by adding an emulsifier, but the presence of emulsifiers affects its water resistance and adhesive strength. Second, the introduction of hydrophilic ionic groups in the molecular framework of polyurethanes allows them to form self-dispersing and self-emulsifying systems. The advantage of this system is that it does not contain emulsifiers, has good film formation, and can be blended with other anionic or cationic polymerization systems to reduce costs. Recent studies on it have been very active, mainly focusing on increasing solids content, increasing drying speed and bonding strength, increasing cross-linking, or introducing polyethers to increase their water resistance. The preparation of ionic water dispersion polyurethane adhesives can be found in relevant patent reports].
2 Hot-melt polyurethane adhesive
Polyurethane hot melt adhesives can be divided into two categories: One is a thermoplastic polyurethane elastomer hot melt adhesive, which mainly utilizes hydrogen bonding in the composition to crosslink, thereby making it have excellent elasticity and strength. The other type is reactive hot melt adhesive, which can be further divided into isocyanate-based prepolymer type (moisture-curing type) and blocked isocyanate-based prepolymer type (latent curing type). One of the major drawbacks of hot melt adhesives currently exists is that liquids with high viscosity at room temperature are even solid. In order to improve its coating performance, it must be heated at 70 ~ 100 °C, to reduce its viscosity to 10 ~ 20Pa? s before use. It has been reported that the use of certain isocyanate monomers to adjust the viscosity of the hot melt adhesive allows it to be applied at lower temperatures or even at room temperature. However, the curing reaction of polyurethane adhesives has a large correlation with temperature. At low temperatures, the reaction rate is slow, which has limited its application. In view of the above shortcomings, foreign literature has pointed out that polyurethane-polyurea as a primer has received good results. The adaptability of the primer and the affinity with the polyurethane adhesive is good, and the adhesive force to the polyethylene, polyester film and other flexible packaging materials is strong. The rate of reaction of urea groups with isocyanate groups is faster than that of hydroxyl groups or water with isocyanate groups, so that the reaction proceeds smoothly at lower temperatures.
The current research on hot melt adhesives is mainly to further improve the initial viscosity and extend the pot life. Originally, such adhesives were generally made of crystalline resins, which were heated to crystallize the molten resin into liquid. After coating, the crystals were crystallized during cooling to produce an adhesive force. The crystallization rate of the resin determines the pot life (open time) and also determines the initial tack. The current idea is to use an amorphous resin, which thickens after cooling and displays initial tack, unlike the crystalline resin that shows initial tackiness after it does not crystallize [6].
3 Solvent type polyurethane adhesive
3.1 One-component solvent-based polyurethane adhesive
The initial viscosity of isocyanate-based, moisture-curing one-part solvent-based polyurethane adhesives is generally lower than that of two-component adhesives. After a polyethylene-polypropylene film (15mm in width) is cured for 4.5h, the bonding strength is 0.6N. , while the two-component is 1.2N or more. However, it is easy to use and also occupies a certain proportion on the flexible packaging composite film. After the one-component polyurethane adhesive is modified with epoxy resin, the initial viscosity will be greatly improved. Polyurethanes such as polypropylene polyols and toluene diisocyanate (TDI) are modified with diethylenetriamine and bisphenol A glycidyl ether (epoxy value 186) to give a solids content of 27.5%. A one-component polyurethane adhesive with a viscosity of 28 s, used for bonding biaxially oriented polypropylene and unstretched ethylene-propylene copolymer films (width 15 mm). The initial adhesive strength and adhesive strength after 9 days are 2.5 N, respectively. And 2.7N.
3.2 Two-component solvent type polyurethane adhesive
3.2.1 cooking-resistant two-component polyurethane adhesive
The disadvantages of polyurethane adhesives are that they are easily hydrolyzed under high temperature and high humidity to reduce the adhesive strength. Therefore, to ensure the performance of the polyurethane adhesive in the high-humidity environment of cooking, special molecular design must be carried out.
To sum up, there are mainly the following methods:
(1) Improve heat and humidity resistance by introducing benzene rings, alicyclic rings, and monomers containing bulky side groups (such as neopentyl glycol), such as the XK-908 high-temperature cooking adhesive developed by Dalian Light Chemicals Co., Ltd. The six-membered alicyclic ring of isophorone diisocyanate and the benzene ring of terephthalic acid are incorporated into the molecule to increase the heat resistance; (2) the macromolecules are crosslinked by the introduction of a trifunctional or higher monomer to increase the resistance thereof. Thermal properties, such as trimethylolpropane; (3) Improve the heat and humidity resistance of polyurethane adhesives by blending modifications. Epoxy resins are used more often. Rigid macromolecular structures and molecular chain energies of epoxy resins are used. The hydroxyl groups reacted with isocyanate groups are cross-linked to increase their resistance to moisture and heat; (4) The hydrolysis-resistant bond can be repaired by adding hydrolysis-resistant stabilizers (eg, carbodiimides, epoxides, etc.) to improve the resistance to moisture and heat. (5) Replacing small hydrophobic monomers with a hydrophobic monomer, such as using long-chain azelaic acid instead of short chain adipic acid when preparing polyesters, can significantly improve its heat and humidity resistance; (6) Protection of ester bonds by side groups by introducing monomers with protective side groups (such as neopentyl glycol) And to improve its moisture and heat resistance; (7) Polyester polyols and polyether polyols are used in the preparation of polyurethane adhesives, making full use of the advantages of good heat resistance of polyester polyols and good water resistance of polyether polyols. Its resistance to cooking performance; (8) By adding "silane coupling agent" (such as KH550, KH560, etc.) to the polyurethane adhesive, the coupling agent forms a hydrophobic protective layer between the adhesive and the adhered material, which can effectively improve Hot and humid;
When the above-mentioned cooking-resistant polyurethane adhesive is used for a composite film containing an aluminum foil layer, special attention must be paid to its resistance to the contents, since the packaged beverage, vinegar, etc. will permeate through the inner film and remain in the aluminum foil layer. Slowly erode the cured coating of the adhesive, resulting in reduced adhesion and even bag delamination. It has been reported in the literature that this can be prevented by adding epoxy resin, silane coupling agent, phosphoric acid, and acid anhydride equal to the adhesive. For ordinary solvent-based polyurethane adhesives, in order to meet the coating process performance, it is generally necessary to dilute to a solid content of 20% to 35%. Therefore, 65% to 80% of organic solvents are volatilized, which not only causes pollution, but also increases costs and Wasted resources. Therefore, in order to solve this problem, ordinary solvent-based polyurethane adhesives should develop to high solid content and low solvent type. According to the volatility requirement of the solvent formulated by the US Environmental Protection Agency, the volatilization solvent for each 3.785L adhesive must not exceed 1.3kg, and the solid content must be higher than 68%. At present, the main way to develop high solid content solvent adhesives is to reduce the molecular weight of the resin. However, the decrease of the molecular weight of the resin reduces the cohesion force and the initial tack strength. At the same time, the increase of the solid content also causes the viscosity change during the process to be large, the pot life is short, and the process performance is deteriorated. Therefore, the development of high solid content polyurethane adhesive should be considered simultaneously from the molecular structure, such as the introduction of polyether polyols and other soft segments in the molecular chain, will help reduce the adhesive viscosity and increase the solid content.
3.2.2 Cold Hardness Solvent Type Polyurethane Adhesive
At present, many polyurethane adhesives have the problem of poor cold resistance. In long-term storage in cold areas (less than 0°C), crystals freeze and crystallize, losing their fluidity and causing inconvenience in use. In the synthesis of polyesters, ether bonds and aromatic rings (diethylene glycol, phthalic acid, etc.) are introduced. These polyesters are reacted with isocyanates to form adhesives and stored at lower temperatures (0 to 4°C). Crystallization does not occur when [25]; can also be added in the adhesive preparation of plasticizers (such as β-methyl-γ-valerolactone) to improve the low-temperature fluidity [26]; also reported in the literature [27] The use of low crystalline polyesters and the use of mixed isocyanates instead of single isocyanates improved the cold flow properties of polyurethane adhesives.
1 Water-based polyurethane adhesive
Water-based polyurethane adhesives were developed in response to the requirements of low toxicity and low pollution products in the 1970s. Water-based polyurethane adhesives are mainly prepared by the following two methods: First, the original polyurethane adhesive is directly emulsified into a stable water-dispersed emulsion by adding an emulsifier, but the presence of emulsifiers affects its water resistance and adhesive strength. Second, the introduction of hydrophilic ionic groups in the molecular framework of polyurethanes allows them to form self-dispersing and self-emulsifying systems. The advantage of this system is that it does not contain emulsifiers, has good film formation, and can be blended with other anionic or cationic polymerization systems to reduce costs. Recent studies on it have been very active, mainly focusing on increasing solids content, increasing drying speed and bonding strength, increasing cross-linking, or introducing polyethers to increase their water resistance. The preparation of ionic water dispersion polyurethane adhesives can be found in relevant patent reports].
2 Hot-melt polyurethane adhesive
Polyurethane hot melt adhesives can be divided into two categories: One is a thermoplastic polyurethane elastomer hot melt adhesive, which mainly utilizes hydrogen bonding in the composition to crosslink, thereby making it have excellent elasticity and strength. The other type is reactive hot melt adhesive, which can be further divided into isocyanate-based prepolymer type (moisture-curing type) and blocked isocyanate-based prepolymer type (latent curing type). One of the major drawbacks of hot melt adhesives currently exists is that liquids with high viscosity at room temperature are even solid. In order to improve its coating performance, it must be heated at 70 ~ 100 °C, to reduce its viscosity to 10 ~ 20Pa? s before use. It has been reported that the use of certain isocyanate monomers to adjust the viscosity of the hot melt adhesive allows it to be applied at lower temperatures or even at room temperature. However, the curing reaction of polyurethane adhesives has a large correlation with temperature. At low temperatures, the reaction rate is slow, which has limited its application. In view of the above shortcomings, foreign literature has pointed out that polyurethane-polyurea as a primer has received good results. The adaptability of the primer and the affinity with the polyurethane adhesive is good, and the adhesive force to the polyethylene, polyester film and other flexible packaging materials is strong. The rate of reaction of urea groups with isocyanate groups is faster than that of hydroxyl groups or water with isocyanate groups, so that the reaction proceeds smoothly at lower temperatures.
The current research on hot melt adhesives is mainly to further improve the initial viscosity and extend the pot life. Originally, such adhesives were generally made of crystalline resins, which were heated to crystallize the molten resin into liquid. After coating, the crystals were crystallized during cooling to produce an adhesive force. The crystallization rate of the resin determines the pot life (open time) and also determines the initial tack. The current idea is to use an amorphous resin, which thickens after cooling and displays initial tack, unlike the crystalline resin that shows initial tackiness after it does not crystallize [6].
3 Solvent type polyurethane adhesive
3.1 One-component solvent-based polyurethane adhesive
The initial viscosity of isocyanate-based, moisture-curing one-part solvent-based polyurethane adhesives is generally lower than that of two-component adhesives. After a polyethylene-polypropylene film (15mm in width) is cured for 4.5h, the bonding strength is 0.6N. , while the two-component is 1.2N or more. However, it is easy to use and also occupies a certain proportion on the flexible packaging composite film. After the one-component polyurethane adhesive is modified with epoxy resin, the initial viscosity will be greatly improved. Polyurethanes such as polypropylene polyols and toluene diisocyanate (TDI) are modified with diethylenetriamine and bisphenol A glycidyl ether (epoxy value 186) to give a solids content of 27.5%. A one-component polyurethane adhesive with a viscosity of 28 s, used for bonding biaxially oriented polypropylene and unstretched ethylene-propylene copolymer films (width 15 mm). The initial adhesive strength and adhesive strength after 9 days are 2.5 N, respectively. And 2.7N.
3.2 Two-component solvent type polyurethane adhesive
3.2.1 cooking-resistant two-component polyurethane adhesive
The disadvantages of polyurethane adhesives are that they are easily hydrolyzed under high temperature and high humidity to reduce the adhesive strength. Therefore, to ensure the performance of the polyurethane adhesive in the high-humidity environment of cooking, special molecular design must be carried out.
To sum up, there are mainly the following methods:
(1) Improve heat and humidity resistance by introducing benzene rings, alicyclic rings, and monomers containing bulky side groups (such as neopentyl glycol), such as the XK-908 high-temperature cooking adhesive developed by Dalian Light Chemicals Co., Ltd. The six-membered alicyclic ring of isophorone diisocyanate and the benzene ring of terephthalic acid are incorporated into the molecule to increase the heat resistance; (2) the macromolecules are crosslinked by the introduction of a trifunctional or higher monomer to increase the resistance thereof. Thermal properties, such as trimethylolpropane; (3) Improve the heat and humidity resistance of polyurethane adhesives by blending modifications. Epoxy resins are used more often. Rigid macromolecular structures and molecular chain energies of epoxy resins are used. The hydroxyl groups reacted with isocyanate groups are cross-linked to increase their resistance to moisture and heat; (4) The hydrolysis-resistant bond can be repaired by adding hydrolysis-resistant stabilizers (eg, carbodiimides, epoxides, etc.) to improve the resistance to moisture and heat. (5) Replacing small hydrophobic monomers with a hydrophobic monomer, such as using long-chain azelaic acid instead of short chain adipic acid when preparing polyesters, can significantly improve its heat and humidity resistance; (6) Protection of ester bonds by side groups by introducing monomers with protective side groups (such as neopentyl glycol) And to improve its moisture and heat resistance; (7) Polyester polyols and polyether polyols are used in the preparation of polyurethane adhesives, making full use of the advantages of good heat resistance of polyester polyols and good water resistance of polyether polyols. Its resistance to cooking performance; (8) By adding "silane coupling agent" (such as KH550, KH560, etc.) to the polyurethane adhesive, the coupling agent forms a hydrophobic protective layer between the adhesive and the adhered material, which can effectively improve Hot and humid;
When the above-mentioned cooking-resistant polyurethane adhesive is used for a composite film containing an aluminum foil layer, special attention must be paid to its resistance to the contents, since the packaged beverage, vinegar, etc. will permeate through the inner film and remain in the aluminum foil layer. Slowly erode the cured coating of the adhesive, resulting in reduced adhesion and even bag delamination. It has been reported in the literature that this can be prevented by adding epoxy resin, silane coupling agent, phosphoric acid, and acid anhydride equal to the adhesive. For ordinary solvent-based polyurethane adhesives, in order to meet the coating process performance, it is generally necessary to dilute to a solid content of 20% to 35%. Therefore, 65% to 80% of organic solvents are volatilized, which not only causes pollution, but also increases costs and Wasted resources. Therefore, in order to solve this problem, ordinary solvent-based polyurethane adhesives should develop to high solid content and low solvent type. According to the volatility requirement of the solvent formulated by the US Environmental Protection Agency, the volatilization solvent for each 3.785L adhesive must not exceed 1.3kg, and the solid content must be higher than 68%. At present, the main way to develop high solid content solvent adhesives is to reduce the molecular weight of the resin. However, the decrease of the molecular weight of the resin reduces the cohesion force and the initial tack strength. At the same time, the increase of the solid content also causes the viscosity change during the process to be large, the pot life is short, and the process performance is deteriorated. Therefore, the development of high solid content polyurethane adhesive should be considered simultaneously from the molecular structure, such as the introduction of polyether polyols and other soft segments in the molecular chain, will help reduce the adhesive viscosity and increase the solid content.
3.2.2 Cold Hardness Solvent Type Polyurethane Adhesive
At present, many polyurethane adhesives have the problem of poor cold resistance. In long-term storage in cold areas (less than 0°C), crystals freeze and crystallize, losing their fluidity and causing inconvenience in use. In the synthesis of polyesters, ether bonds and aromatic rings (diethylene glycol, phthalic acid, etc.) are introduced. These polyesters are reacted with isocyanates to form adhesives and stored at lower temperatures (0 to 4°C). Crystallization does not occur when [25]; can also be added in the adhesive preparation of plasticizers (such as β-methyl-γ-valerolactone) to improve the low-temperature fluidity [26]; also reported in the literature [27] The use of low crystalline polyesters and the use of mixed isocyanates instead of single isocyanates improved the cold flow properties of polyurethane adhesives.
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